There are detailed rules in section 3 of the BCAP Code.
Comparisons with other advertisers and products/services are acceptable where they are factual, rather than denigratory.
Comparisons must be fair and not misleading. Tone of voice is important for radio advertising, e.g. sneering voiceovers are unacceptable.
For comparative claims to be acceptable (e.g. ‘X brand washes whiter’), the advertiser must have demonstrated satisfactorily that its brand has been tested like-with-like against all other competitors and achieved superior results. Otherwise, the comparative claim needs to qualify how many or which other brands have been tested alongside the advertised brand and been shown to be less effective (e.g. ‘in tests against the three leading brands, X washed whiter’).
Claims based on subjective criteria have the potential to mislead. Any claims must be supported by reliable evidence and must be appropriately qualified in the advertisement.